In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].

(CNN) Hunkered in a Vietnam bunker, with rockets and mortar shells raining down, the two Marines made a pact. It was New year's Eve, 1968. And they promised that if they survived the war and made it home alive, the two would call each other every New Year's Eve. And they did. For nearly half a century, Master Sgt. William Cox and Sgt. James Hollingsworth checked up on each other. But on Sunday, Cox will ring in the New Year alone. His friend "Hollie" died in October. He was 80 years old. A promise made Cox and Hollingsworth flew missions together as helicopter door gunners. On the ground in Vietnam, Hollingsworth helped maintain the choppers. Cox kept track of ammunition.William Cox and James Hollingsworth make pact in Vietnam bunkerFor first time in half century, old marine marks New Year's without buddy 
 Question:He ended the eulogy with a phrase the two used to repeat: "_, you keep 'em flying, and I'll keep 'em firing."
Hollie